The ability of Pseudomonas aeruginosa strains to adapt under adverse conditions may explain in part its virulence capabilities. One example of this adaptibility is their response to the bactericidal activity of pooled normal serum (PNHS). Whereas most strains isolated from the sputum of patients with cystic fibrosis (CF) are serum sensitive (serS) and deficient in lipopolysaccharide (LPS) O side chains, those isolated from non-CF patients are usually serum-resistant (serR). To examine the nature of serum resistance in P. aeruginosa strains, 6 serR derivatives were isolated from 6 serS parental strains. The initial characterization of 3 of these mutants suggests that serum resistance may be attributable to LPS O-side chain length (which is longer in serR than in serS strains), and in one strain, the absence of an outer membrane (OM) protein (OMP). Furthermore, using purified human complement components, it was determined that the serum resistance of one strain, 144M-SR, is not due to its inability to activate complement effectively, but rather is due to the failure of the assembled terminal complement C5b-9 to insert stabily into the outer membrane of this strain. Using 4 pairs of slime-producing strains (4 serS, short O-chain strains and their 4 serR, long O-chain counterparts), plus 10 additional serS or serR strains isolated from CF and non-CF patients, this grant application proposes to 1) identify how PNHS kills serS P. aeruginosa strains, 2) determine how serR strains resist the bacterial activity of PNHS, 3) compare the pathogenicity of serS and serR strains in a chronic rat lung infection model, and 4) examine the susceptibility of serS and serR strains to opsonophagocytosis by alveolar macrophages. These specific aims will not only define the mechanisms of serum sensitivity/resistance in P. aeruginosa, but will also provide valuable information concerning the OM components of CF and non-CF patient isolates and their interaction with antibodies and complement. In addition, the comparison of serS, short O-chain strains and their serR, long O-chain counterparts in regards to pulmonary defenses (both in vitro and in vivo) may help explain why CF isolates persist in the lungs of CF patients. As a result, these investigations should provide directional guidelines for future studies geared toward either preventing or controlling infection with Pseudomonas aeruginosa.